Symmetry and positivity properties (see [1]}) :
\(\forall n\in \mathbb {N},\quad \forall x>0,\quad I_{-n}(x)=I_{n}(x)\in \mathbb {R}_{+}^{*}\quad \mbox{ and }\quad K_{-n}(x)=K_{n}(x)\in \mathbb {R}_{+}^{*}.\) 
